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<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'>Hi,</span></font></p>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'>My name is Sushant Rewaskar. I am a graduate student at UNC Chapel
Hill. I have been looking at how the windows OS estimated the RTO (Retransmission
Time Out) but looks like it is not such a simple problem. </span></font></p>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'> </span></font></p>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'>I ran a series of experiments with constant RTT as well as with
variable RTT to try and figure out what equations (of the form RTO_timer = a
*srtt + B *rttvar) does windows use. Turns out, the equation is not that
simple. Add to this the unknown clock granularity and this becomes a touch
problem. </span></font></p>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'> </span></font></p>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'>I am seeking information or/and ideas as to how one can estimate the
equation(s) used by windows. Below are details of how we approached this
problem.</span></font></p>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'> </span></font></p>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'>To create a controlled environment we set up two machines in the lab.
One of the test machines was windows (with XP pro) and the other was a FreeBSD
machine. We used tbit [http://www.icir.org/tbit/] to request a webpage from the
windows machine. We allowed the connection to transfer 250 packets (allowing
the connection to enter congestion avoidance and allow the RTO estimator
equation to converge). The above experiments were first run with a constant
RTT. (We varied the values from 50, 100, 150, 200, 400, 750, 1000, 1200, 1400
ms). Under the constant RTT situation the rttvar should be reduced to a very
low value. In this case the RTO estimator should be a direct function of srtt
and maybe a constant. The empirically derived RTO value would be affected by
the clock granularity of the system. For e.g. if the clock granularity is
100ms. The actual time when the RTO timer is triggered can be offset by 100ms
on either side of the actual time. To take care of this we ran the tbit
experiment 1000 times for each of the 9 RTT values and took the median as the
probable RTO value for that RTT. For constant RTT we assumed that the RTO
estimation equation was of the form. RTO = a*srtt + C. I used a matlab linear
equation solver to estimate the constants a and C. the results I got varied
with the experiments and did not appear intuitively (got constants like 2.7 for
“a” and similar non integer for “C”)</span></font></p>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'> </span></font></p>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'>We repeated these experiments with variable RTT (the same base values
as above with a 25% random change in the RTT). We hoped to have obtained
“a” from above and then estimate “b” for the following
RTO equation (RTO = a * srtt + b rttvar).</span></font></p>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'> </span></font></p>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'>Anyway, any ideas are welcome. </span></font></p>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'> </span></font></p>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'>Take care,</span></font></p>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'>Sushant Rewaskar </span></font></p>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'>UNC Chapel Hill</span></font></p>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'><a href="http://www.cs.unc.edu/~rewaskar"><span lang=PL>www.cs.unc.edu/~rewaskar</span></a></span></font></p>
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